The present invention relates to spinal implants, tools for the insertion of spinal implants, and methods of use thereof, and more particularly, to an implantable modular fusion cage for promoting fusion of adjacent vertebral bodies.
Degenerative changes in the spine can cause the loss of normal structure and/or function. The intervertebral disk is one structure prone to the degenerative changes associated with wear and tear, aging, and even misuse. Over time the collagen (protein) structure of the intervertebral disk weakens and may become structurally unsound. Additionally, the water and proteoglycan (the molecules that attract water) content decreases, thereby narrowing the space between the adjacent vertebrae, which can result in nerve root compression and pain. These changes can lead to the disk""s inability to handle mechanical stress.
One form of treatment available for degenerative disk disease is spinal fusion surgery, which involves the surgical removal of a portion or all of an intervertebral disk followed by fusion of the adjacent vertebrae. A prosthetic device, e.g. a fusion cage, is usually placed between the two adjacent vertebrae to fill the space left by the removed disk and to allow bone to grow between the adjacent vertebrae.
Spinal fusion procedures can present the surgeon with several challenges, especially where the disk is severely degenerative. When the natural disk is removed, the adjacent vertebral bodies collapse upon each other thereby requiring the bodies to be separated to enable placement of the prosthesis. However, separation or distraction of the vertebral bodies beyond a certain degree can result in further injury or damage to the vertebrae. Conversely, where the disk is severely degenerative, the narrow disk space and lack of elasticity between the vertebrae can hinder the surgeon""s ability to separate the vertebrae to a height sufficient to enable placement of the prosthesis.
To overcome some of these problems, specialized tools have been developed to facilitate the placement of disk prosthesis between adjacent vertebral bodies of a patient""s spine. Among the known tools for performing such procedures are spinal distracters, e.g. spreaders, and insertion devices. In general, the spreader is placed between adjacent vertebrae, and then used to pry the vertebrae apart. Once the space between the vertebral bodies is sufficient to enable placement of a prostheses, the prosthetic device can then be inserted, either manually or with an insertion tool, into the space to hold the adjacent vertebrae apart. Typically, cancellous bone is packed in and/or around the cage to promote fusion of the adjacent vertebrae.
While most spreader devices are effective to assist surgeons with the placement of disk prosthesis, the use of such tools can prove cumbersome. For example, insertion of a spreader device into the limited disk space can cause fracture of a vertebra. Moreover, once inserted, the spreaders can cause over-distraction of the vertebral bodies, or can hinder placement of the prosthesis. In the presence of degenerative disease or chronic changes where the disk space has become narrow, it can be difficult to maintain an adequate interbody height and, at the same time, insert and position the implant. Over-insertion, or under-insertion of the prosthesis can lead to pain, postural problems and/or limited mobility or freedom of movement.
Despite existing tools and technologies, there remains a need for a device to facilitate the safe and accurate insertion of a disk prosthesis between adjacent vertebral bodies while minimizing the risk of further injury to the patient.
The present invention provides a modular fusion cage for promoting fusion of adjacent vertebral bodies, and a tool for the insertion of the fusion cage. The implant is effective for maintaining an adequate interbody height between adjacent vertebrae during insertion, and therefore is particularly useful for patients with degenerative disk disease. In addition, the modularity of the implant facilitates the safe and accurate insertion of the implant between adjacent vertebrae. The modular fusion cage of the invention is particularly useful as an anterior fusion cage.
In general, the modular fusion cage includes a ramp member and a ring member, each of which has superior and inferior bone contacting surfaces, and anterior and posterior sides. The ramp member is adapted to be positioned between adjacent vertebral bodies, and the ring member is slidably matable with the ramp member.
In one embodiment, the ring member is generally C-shaped with an opening that is preferably disposed on the posterior side of the ring member. The opening represents an interruption in the continuity of the annular walls of the ring member and is effective for receiving the ramp member. The ring member can also include a substantially wedge-like posterior tip formed between the superior and inferior surfaces of the ring. The ramp member can include transverse sides connecting the superior and inferior surfaces, each of which can be adapted to conform to the opening in the ring member.
In one embodiment, the ring member further includes first and second rib structures that extend substantially parallel to each other between the posterior and anterior sides of the ring member. The first and second rib structures are each separated from the side walls of the ring member, and each other, by openings in which bone growth promoting materials, e.g. cancellous bone, can be placed.
The superior and inferior surfaces of the ring and/or ramp members of the modular fusion cage of the present invention can have any shape, and can include bone engaging surface features (e.g. teeth or ridges) formed on at least a portion thereof. Preferably, these surfaces are generally shaped and contoured to conform to and to be complimentary to the surfaces of the vertebral bodies that they are intended to contact. By way of non-limiting example, the superior and inferior surfaces of the ring and/or ramp members can have a convex shape, or can have a height extending between the superior and inferior surfaces that is greater at the anterior side than at the posterior side of the ring/ramp member.
As a modular fusion cage, the components that form the cage are assembled within the patient""s body during a surgical procedure. The ramp member is positioned first and the ring member is subsequently installed and mated to the ramp member. To ensure proper mating, the ring and/or ramp members include alignment features that assist in aligning the ring member with the ramp member during insertion. The alignment guide can be, for example, a tongue and groove formed on the transverse side walls of the ramp member and the opening in the ring member. The ring and ramp members can optionally include other features, such as a locking mechanism effective to lock the ring and ramp members together, or a handle removably attached to the ramp member for inserting the ramp member into the disk space, and optionally for inserting the ring member around the ramp member.
In yet another embodiment, a spinal implant system is provided having a modular fusion cage and an insertion tool for implanting the modular cage within the vertebral disk space. The modular fusion cage includes a ramp member and a ring member, each having posterior and anterior faces, and superior and inferior bone contacting faces. The posterior face of the ring member defines an opening for receiving the ramp member, and the anterior side of the superior and inferior faces each have at least one groove formed therein for receiving the insertion tool. The insertion tool includes first and second opposed grasping elements movable between an open position and a closed position, and matable with the at least one groove formed in each of the superior and inferior surfaces of the ring member. First and second opposed actuating members are mated to the first and second grasping elements and are effective to move the first and second grasping elements between the open and closed positions.